Commonly, alkali metal halide brines, such as sodium chloride brines and potassium chloride brines, are electrolyzed in an electrolytic cell wherein a liquid permeable diaphragm divides the cell into an anolyte compartment with an anode therein and a catholyte compartment with a cathode therein to produce chlorine, hydrogen, and aqueous alkali metal hydroxide. Asbestos has been the most common diaphragm material, but has suffered from relatively short lifetimes and from environmental concerns. Numerous efforts have been made to improve the lifetimes and performances of asbestos diaphragms. For example, according to U.S. Pat. No. 3,991,251, asbestos diaphragms can be strengthened by the reaction between asbestos and sodium hydroxide at temperatures from about 110.degree. C. to 280.degree. C. Other patents describe strengthening asbestos diaphragms by addition of polymeric resins, e.g., fluorine-containing polymers, to bind the asbestos diaphragms. See U.S. Pat. Nos. 4,065,534; 4,070,257; 4,142,951; and 4,410,411.
Asbestos-free microporous diaphragms have been produced by sintering materials such as polytetrafluoroethylene (PTFE) and a particulate pore forming additive followed by subsequent removal of the additive, as shown by for example U.S. Pat. Nos. 3,930,979, 4,098,672 and 4,250,002. U.S. Pat. No. 4,036,729 describes depositing discrete thermoplastic fibers of, e.g., a fluorinated hydrocarbon, from an aqueous medium containing acetone and preferably a fluorocarbon surfactant onto a cathode screen for use as a diaphragm in electrolytic cells. The deposited fibers form an entanglement or network which does not require bonding or cementing. Unfortunately, such polyfluorocarbon diaphragms generally are hydrophobic, i.e., difficult to wet with water. This hinders passage of an aqueous electrolyte through the diaphragm, and results in high cell voltages, particularly in comparison to asbestos-based diaphragms under similar cell conditions.
U.S. Pat. No. 4,482,441 describes codeposition of fibrils of a hydrophobic organic polymer, e.g., a copolymer of tetrafluoroethylene and perfluoropropylene, and a hydrophilic group IIA metallic oxide, e.g., magnesium oxide particles, from an alkaline brine containing sodium hydroxide, sodium chloride and a polyethyleneimine-based retention agent onto the cathode of a cell. Such a deposited diaphragm may also include a surface active agent, e.g., a fluorinated surface active agent.
Finally, U.S. Pat. No. 4,606,805 describes a diaphragm containing as its principal particulate ingredient an inorganic material such as talc, a metal silicate, an alkali metal titanate, an alkali metal zirconate or a magnesium aluminate, along with both polytetrafluoroethylene fibers and polytetrafluoroethylene particulates.
Clearly, further developments are constantly sought whereby diaphragms may achieve improved performance in terms of cell voltages while exhibiting excellent wettability by aqueous electrolytes.